This invention relates to a polarising filter and its manufacturing method.
Polarising filters are subject to numerous optical applications, either to form polarised luminous fluxes or to select within a luminous flux carrying various polarisations, one of them being of particular interest.
Certain transparent materials, at least in a given spectral domain, exhibit polarising properties and the first polarising filters have been made with such materials. Linear polarisations are considered generally.
More recently, numerous techniques have been developed and in particular (for example in the U.S. Pat. No 3,046,839), it has been suggested to make structures including metallized grids for the manufacture of polarisers.
The aim of the invention is to provide such a filter and its manufacturing method, with high whole transmission coefficient and having a very high ratio between the polarised luminous flux in one direction whose transmission is maximum and the polarised flux in the direction perpendicular whose attenuation is the strongest. Preferably, this ratio should be uniform over an extended spectral domain.
This invention relates therefore to a transmission-polarising filter comprising a substrate and a partially metallized grating, carried by said substrate.
According to the invention, the grating is a holographic grating having a profile with facets of slopes that are alternately positive and negative, whereas one of the sets of these facets with positive or negative slopes, is partially metallized.
In various embodiments, each exhibiting its specific advantages:
the profile of the grating is a sine wave,
if the central wavelength of the useful bandwidth of the polariser is xcex, the pitch of the grating is about xcex/2,
the depth of the lines of the grating is in the order of xcex/10,
the number of lines in the grating per mm ranges between 3,000 and 5,500, the modulation depth of the lines ranges between 30 and 70 nm, the metallic layer has a maximum thickness ranging between 10 and 40 nm.
The invention also relates to a manufacturing method of such a polarising filter, comprising the following steps:
manufacture of a holographic grating on a substrate,
coating of a metallic layer with uniform thickness on the holographic grating,
partial attack of the metallic layer by ionic machining.
In various preferred embodiments, each exhibiting their specific advantages:
the metallic layer is a silver layer,
ionic machining is performed while tilting the substrate with respect to the ionic beam to 30-80xc2x0,
the grating is formed in a layer of epoxy resin,
the grating is formed in a photoresist layer.